As is well known in the art, cardiac pacemaker systems provide for delivery of timed pacing pulses designed to cause the myocardium of the heart to contract, or beat, so as to compensate for the inability of the heart to provide normal beating. Stimulation pulses, or pacing pulses generally are set at a programmable energy level, involving both pulse width (duration) and amplitude, so as to provide pulses of sufficient energy to actually stimulate the heart into a contraction. When a delivered pace pulse is successful in so stimulating the heart to contraction, it is said to have "captured" the heart, whereas failure to stimulate the heart is described as "loss of capture" (LOC). It is thus an important function of the pacemaker to control the pulse energy so that it is sufficiently above the stimulation threshold to generally ensure capture, but yet avoid excessively high energy pulses and thereby minimize battery depletion.
As is well known, the stimulation threshold for a patient, in both the atrium and the ventricle, can fluctuate both short term and long term. Because stimulus threshold may increase for a variety of reasons, it has become state of the art to periodically conduct a threshold tests, and to readjust the pulse energy in accordance with any newly determined threshold. The pacing literature provides many examples of such threshold tests. An early example of an automatic threshold tracking pacemaker is found in U.S. Pat. No. 3,920,024 (Bowers), where the stimulus energy is initially set at a high enough energy to ensure capture, and then is reduced by successive increments until capture is lost. In this technique, capture must be detected directly by sensing the evoked signal, which remains a difficult task. Further, when the pulse energy is dropped low enough so that capture is lost, a back-up pulse must be delivered so as to avoid skipping a heartbeat.
Due to the inherent difficult of sensing an evoked depolarization at or near the site of delivering a stimulus pulse, a wide variety of different approaches have been taken in an attempt to reliably determine when a delivered pacing pulse has not resulted in capture. See U.S. Pat. No. 5,601,615, Medtronic, Inc., which presents a summary of such approaches that are to be found in the patent literature. These approaches include separate electrodes or electrode systems and amplifiers for sensing evoked responses; physiologic sensors for determining mechanical changes in the heart or changes in the blood when the heart is captured, etc.
This invention addresses primarily the need in a DDD, or DDD(R) pacing system to determine on an ongoing basis whether a delivered atrial pacing pulse has result in capture of the atrium. The standard technique with regard to this problem is to undertake a test, either initiated from an external programmer or initiated automatically by an implanted pacemaker, to obtain information to determine whether an atrial pace pulse achieves capture. See, for example, U.S. Pat. No. 5,476,486, where the amplitude of the atrial pacing pulse is decreased progressively, while the following R wave is monitored; the absence of an R wave indicates loss of atrial capture. However, if the patient has a high degree of AV block, and ventricular pacing is required, this method cannot be used. Note also the aforementioned U.S. Pat. No. 5,601,615, which employs a test procedure for delivering atrial pacing pulses at an early atrial escape interval which precedes the anticipated natural atrial contraction, and looking for a ventricular sense within the latter portion of the AV delay interval. If the ventricular sense is not found within such latter portion, then failure to capture is assumed. Other examples from the prior art require special circuitry and/or leads with specific electrode arrangements designed to detect the occurrence of an evoked response within an anticipated time following delivery of an atrial pace pulse.
What is needed in the art is a pacemaker with the capability of reliably detecting when a delivered atrial pace pulse has evoked a P wave, without requiring special tests or additional components or circuitry which consume energy and add to the bulk and cost of the system. Specifically, there is a substantial need for a pacemaker which utilizes already available information and thus requires no special circuitry or test procedures, for reliably determining on an ongoing basis when a delivered atrial pacing pulse has not captured the atrium. The need is particularly evident in the specific situation where, following lack of capture but before timeout of the AV interval, a spontaneous atrial contraction takes place which the pacemaker does not sense. In such a situation a ventricular pace pulse is delivered asynchronously with respect to the intrinsic atrial contraction. If such asynchronous pacing is permitted to continue, the patient has lost the important benefit of synchronous pacing.